There is increasing interest in producing fuel ethanol or other fermentation products from cellulosic feedstocks. A particular feedstock of interest is cellulosic material remaining after sugar cane processing. The insoluble portion of sugar cane, which is known as bagasse, has a high polysaccharide content and could be suitable for conversion to ethanol or other fuels or chemicals. The leaves and tops of sugar cane can also serve as a source of sugar for conversion into such products.
One process proposed for producing a fermentation product, such as ethanol, from cellulosic materials derived from sugar cane is to carry out a pretreatment, followed by enzymatic hydrolysis of the cellulose to glucose. The pretreatment generally disrupts the fiber structure of the cellulosic feedstock and increases the surface area of the feedstock to make it accessible to cellulase enzymes. The pretreatment can be performed so that a high degree of hydrolysis of the xylan and only a small amount of conversion of cellulose to glucose occurs. The cellulose is hydrolyzed to glucose in a subsequent step that uses cellulase enzymes. Other pretreatment processes, such as certain alkali pretreatments, do not hydrolyze or result in limited xylan hydrolysis. Moreover, it is possible to hydrolyze both xylan and cellulose using more severe chemical treatment, such as concentrated acid hydrolysis.
Regardless of the method for producing fermentable sugar, the addition of water to the incoming feedstock to form a slurry is often carried out to facilitate the transportation and mechanical handling of the cellulosic feedstock. The slurry typically consists of cellulosic feedstock pieces or particles in water at a consistency of about 1 to about 10 wt % undissolved dry solids, as feedstock slurries can be more easily pumped when the dry solids content falls within this range.
However, for cellulosic conversion processes to be more economical, it would be desirable for them to operate at lower water content. The processing of feedstock of low water content has numerous advantages in various stages of the process, one of which is reduction in equipment size, which, in turn, reduces capital cost. Further benefits of low water content include reduced energy consumption including reductions in costs for pumping, heating, cooling and evaporating. Moreover, water usage costs can be reduced, which is especially advantageous in arid climates where water is at a premium.
Another problem associated with converting lignocellulosic feedstocks to ethanol and other fermentation products is the presence of ash in the in-coming feedstock, which contains inorganic salts, silica and other components. The presence of ash in lignocellulosic feedstocks, particularly inorganic salts such as potassium salts, is known to increase chemical consumption due to its buffering effect. It is known to remove ash from feedstocks such as wheat straw by leaching with an aqueous solution prior to chemical treatment as described in U.S. Pat. No. 7,901,511 (Griffin et al.). While leaching can reduce chemical demand, it may consume significant amounts of water. Thus, ash is often not removed from feedstocks such as wheat straw prior to processing due to the cost associated with its removal.